Monoalkyl pentraborane-11 and process for its preparation



United States Patent 3,391,194 MGNOALKYL PENTABORANE-ll AND PROCESS FGR ITS PREPARATION Emil A. Lawton, Columbus, Ohio, Earl A. Weilmuenster, Kenmore, N.Y., and Arthur Levy, Worthington, Ohio, assignors, by direct and mesne assignments, to The Battelle Development Corporation, Columbus, Shin, a corporation of Delaware No Drawing. Filed Oct. 12, 1955, Ser. No. 540,143 7 Claims. (Cl. MIL-606.5)

This invention relates to fuels, and more particularly, to fuels in which boron is an essential constituent.

Military aircraft require a fuel with as high a heating value per unit of weight as possible to maximize range. Liquid hydrocarbons, such as gasoline, have a lower heating value per unit of weight than is desirable, particularly for rockets and guided missiles. Lighter hydrocarbons have a higher heating value per unit of weight, but are volatile under normal conditions of temperature and pressure, necessitating high-pressure storage tanks. A liquid fuel having low volatility and high heating value per unit weight would therefore be more desirable than hydrocarbon fuels for military aircraft.

An object of the present invention is to provide a fuel suitable for military aircraft, such as rockets, jet airplanes, and guided missiles.

A further object of the present invention is to provide a liquid fuel having low volatility and, at the same time, a high heating value in excess of the heating value of hydrocarbon fuels of comparable volatility.

A still further object of the present invention is to provide a fuel which remains liquid at extremely cold temperatures, below -60 C.

According to this invention the addition products of pentaborane-11 and mono-olefins having 2 to 4 carbon atoms are prepared by direct reaction of pentaborane-ll and an olefin in the presence of hydrogen or other inert diluent gas, such as nitrogen or mixtures of hydrogen and nitrogen. The reactions are carried out at low temperatures, generally within the range from about 25 C. to about 30 C. and preferably at about 0 C. The resulting addition products are liquids which have low vapor pressures at 0 C., remain liquid at 80 C., and have heating values in excess of the heating value of methane. These pro-ducts are suitable as fuels for various military aircraft such as jet airplanes, rockets and guided missiles.

The pentaborane-1l starting material may be either pure pentaborane-ll, B H or may be a mixture of B H with B H A mixture of B H and B H containing more than 80 percent of the former can be prepared easily by pyrolysis of diborane at an elevated temperature of about 100-130" C. This method is described in an article by AB. Burg and H. I. Schlesinger, in the Journal of the American Chemical Society, vol. 55, p. 4009 (1933). The pyrolysis products of this reaction are B H B H 13 1-1 and B H plus minor amounts of nonvolatile boron hydrides. The B H B l-I and nonvolatile products are easily separaed from the mixture of B H and B E-I Since there is little difference in the vapor pressures of B H and B H these compounds are not readily separated by fractionation. However, it has been found that B H does not interfere with the reaction according to the present invention and acts as an insert solvent for the olefin. The pentaborane starting material used in the reactions according to the present 3,391,104 Patented July 2, 1968 invention can contain about 10 to 20 percent by weight of B H and the balance B H although of course pure B H can also be employed as a starting material.

The hydrocarbon reagent according to the present invention is a mono-olefin having 2 to 4 carbon atoms. Ethylene and propylene have been found particularly useful as reagents. The butylenes, such as butene-l, are also suitable reagents. Mixtures of mono-olefin hydrocarbons having from 2 to 4 carbon atoms can also be utilized, where desired.

The reaction according to the present invention is advantageously carried out batchwise but may also be carried out continuously. Pentaborane--l1 is present as a liquid, and the olefin as a gas in the reaction vessel at reaction temperature. The presence of hydrogen or other inert diluent gas is essential to obtain a liquid product. Low temperatures are necessary to carry out the reaction of the present invention. A temperature of about 0 C. is preferred. Higher temperatures, up to about 30 C., are permissible, although the yield is lower.

The total pressure of the reaction system is not critical, although in general an increase in hydrocarbon pressure is attended by an increase in yield and reaction rate. Suitable initial pressures for carrying out the reaction of the present invention range from subatmospheric pressures in the order of about one-third of an atmosphere up to slight superatmospheric pressures, for example, up to about 25 psig. The reaction can be carried out quite satisfactorily at atmospheric pressure.

Pentaborane-ll and an olefin are charged to the reaction vessel in proportions such that the pentaborane-ll and the olefin present in a 1:1 molar ratio or somewhat higher or lower, the molar ratio of pentaborane-l1 to olefin generally being within the range from 1:3 to 3:1. Either reagent may be introduced into the reaction vessel first, or the two may be charged simultaneously.

Hydrogen or other inert diluent gas is charged to the reaction vessel after both the olefin and pentaborane-11 have been introduced. Although hydrogen or other inert diluent gas does not undergo reaction, its presence is essential. In the absence of hydrogen or other inert diluent gas, a solid, gel-like material is formed instead of the desired liquid product. The amount of hydrogen or other inert diluent gas present does not have any substantial effect on either the reaction rate or the quality of the product. In general, from 0.02 to 10 moles or hydrogen or other inert diluent gas will be introduced into the reaction zone, based upon the total moles of pentaborane-11 and hydrocarbon introduced.

The reaction is preferably carried out at a temperature of about 0 C. At this temperature the olefins which may be used in this process are gases and the pentaborane reagent comprising B H and B H is a liquid. The greater portion of the olefin is in the gas phase, although part is dissolved in the pentaborane reagent. The reaction of this invention appears to take place in the liquid phase between the olefin and B H Normal pentaborane (B H does not enter into the reaction, and merely acts as an inert solvent. To maximize the reaction rate, the partial pressure of olefin should be as high as possible.

The reaction time according to the present invention is quite lengthy, ranging from about 3 to slightly over hours. However, the best yields are obtained when the reaction is made to proceed in the shortest possible time. The partial pressure of olefin has already been indicated as a prime factor in influencing the reaction time. The reaction vessel is maintained at reaction temperature as long as the adduct of the olefin and pentaborane-11 is being formed.

The reaction products may be cooled with liquid nitrogen to 196 C. to condense all materials except hydrogen or other inert diluent gas and to stop further reaction when the desired reaction is complete. The hydrogen gas or other inert diluent gas is removed. The reaction vessel is then warmed up by exposure to room temperature. This results in vaporization of the product of this invention as well as most of the other material in the reaction vessel.

The products may be separated by fractionation. Conventional high-vacuum techniques can be used. According to one mode of operation, the products are withdrawn from the reaction vessel under vacuum and passed successively through a pair of condensate traps maintained at 80 C. and 196 C., respectively. The desired olefin-B H adduct as well as unreacted B H and 13 1-1 are condensed in the first trap, while other materials, particularly the unreacted olefin, diborane, and organoboron compounds more volatile than the olefin-B H adduct which are formed by side reactions, pass through the first trap and are condensed in the second trap. The condensate from the first trap, which is maintained at -80 C., is then further fractionated, preferably several times, at 30 C. to -60 C. to obtain the adduct as a substantially pure product in the liquid state. Unreacted pentaborane-11 is collected as a vapor from this fractionation.

having more than 4 carbon atoms have an undesirably low boron-to-carbon ratio. The heating values of the adducts of pentaborane-11 with olefins having 2 to 4 carbon atoms per molecule are quite high, always in excess of 21,000 B.t.u. per pound. The heating values of the adduct of pentaborane-ll with ethylene and propylene, both experimentally determined and theoretical values, are shown in Table I below. As would be expected, the theoretical or calculated heating value of the ethylene adduct is slightly higher than that of the propylene adduct.

The reaction product of ethylene with pentaborane-11 is pyrophoric at ordinary temperatures. When this product is exposed to a large quantity of air at 20 C., virtually instantaneous flash combustion takes place, leaving white solid boric oxide as a combustion product. The reaction product of propylene with pentaborane-11, on the other hand, is not pyrophoric at ordinary temperatures. This product is oxidized rapidly, however, in air at 20 C.

The products of the present invention are not stable above 0 C. They gradually break down into diborane, a noncondensable gas which is presumably hydrogen, and other products which, for the most part, were not identified. Decomposition increases with temperature.

Properties of the adducts of B H with ethylene and propylene are summarized in Table I below. Results of all runs are given where more than one determination has been made.

TABLE I.PROPERTIES OF THE ADDUCTS OF BsHn WITH E'IIIYLENE AND PROPYLENE Analysis (percent/wt.) Molecular Weight Vapor Density, Heat of Combustion,

Pressure, g./ml., B.t.u./lb. 0 0. mm. 0 C. Compound Observed Theoretical Observed Theoretical Observed Theoretical B C H B C H cinmsrrm 58 28.1 15.9 58.1 25. 8 16.1 {93 93 1. 5 {0. 77 22,000 25,300

.96 0.73 21, 700 C H B H [53.4 32. 4 33.6 16-0 107 0. 5 0.68 26, 400 24,450

The pressure, temperature, and equipment used in fractionating the reaction products may be varied within wide limits. The temperature should not be allowed to rise above 0., preferably not above 0 C., to avoid decomposition of the olefin-B H adduct and unreacted B H Considerable variation in temperatures in individual fractionation units is permissible. Other types of fractionating equipment, such as partial condensers, may be used in lieu of condensate traps.

Yields of the desired product up to 95 percent based on the weight of B H reacted can be obtained. The yields are slightly lower based on the amount of olefin consumed. This is probably due to side reactions, particularly polymerization.

The products obtained according to the present invention are addition products of the olefin and pentaborane- 11. The ethylene adduct has the empirical formula C B H and the propylene adduct has the empirical formula C B H In each case the compound is an addition product having a 1:1 ratio of olefin to B H The products of the present invention are clear liquids which have a low vapor pressure. The freezing and boiling points have not been determined, but the products are known to be liquid at -80 C. The vapor pressure of the propylene-pentaborane-ll adduct is about 0.5 mm. at 0 C. The products of the present invention, being in liquid form and having a low vapor pressure, are very convenient to use as fuels. Furthermore, the heating value of these compounds is much higher than that of hydrocarbon fuels of comparable volatility. The higher heating value as compared to hydrocarbon fuels is due to the fact that boron has a much higher heat of combustion than does carbon. The heat of combustion of boron is 25,660 B.t.u. per pound. To attain the highest possible heating value, the ratio of boron to carbon should be as high as possible. Adducts of pentaborane-l1 with mono-olefins Elemental analyses in the foregoing table were determined using the procedure described in an article by H. C. Brown, H. I. Schlesinger, and A. D. Burg, Journal of the American Chemical Society, vol. 61, p. 673 (1939).

This invention will be further illustrated with respect to specific examples. All gas volumes are the volumes at standard temperature and pressure (0 C. and one atmosphere).

Example I A mixture of 100 cc. of ethylene gas and 113 cc. of pentaborane gas reagent (at least B H balance B I-I was introduced into a reaction tube having a volume of 194 cc. and condensed at 196 C. by external cooling with a liquid nitrogen bath. The pentaborane reagent was prepared by the method of Burg et al. de scribed above. Then 52 cc. of hydrogen gas, which had been freed of oxygen and dried at --196 C., were introduced into the vessel.

The reaction tube was allowed to warm up under ambient conditions, and the temperature was then held at 0 C. for 50 hours. During this time the reaction continued. The initial pressure was 594 mm. of mercury. As the reaction proceeded, the pressure dropped to a final pressure of 256 mm. The reaction was terminated at the end of 50 hours by freezing the reaction mixture with liquid nitrogen at -196 C. Hydrogen, which was the only uncondensed substance, was found to have a volume of 53 cc. and then was withdrawn from the reaction tube. The initial and final volumes of hydrogen are the same within the limits of experimental error.

The condensed substances in the reaction tube were allowed to warm up and were withdrawn as vapors from the reaction tube through a vacuum line having a pair of condensate traps maintained at 80 C. and 196 C. by a Dry Ice-solvent mixture and liquid nitrogen, respectively.

A mercury diffusion pump located beyond the second condensate trap maintained an absolute pressure of less than 10 mm. exclusive of the vapor pressure of mercury, at the pump inlet.

The condensate in the trap maintained at 80 C. was refractionated eight times. In each refractionation the condensate was allowed to warm up and the vapors passed successively through condensate traps maintained at 60 C. and 80 C., respectively. High vacuum was maintained with a mercury diffusion pump. The adduct of ethylene and pentaborane-ll was condensed at --60 C. in the first trap, and unreacted pentaborane-ll was condensed in the second trap. The adduct fraction was refractionated in the manner described, until a pure adduct free of pentaborane-ll was obtained after eight fractionations. The pure adduct fraction collected during the last fractionation weighed 0.226 g. This represents a yield of 90 percent based on the amount of pentaborane-ll consumed. Pentaborane-ll and B H were recovered in the condensate traps at 80 C. during fractionation and had a vapor volume of 47.5 oc.

The condensate in the trap maintained at --l96 C. had a volume of 14.6 cc., and analyzed 23% unreacted B 11 and B H 13% unreacted ethylene, 45% diborane, and the remainder unidentified organoboron and other organic compounds.

Example II The procedure of Example I was followed except that the starting materials, reaction conditions, product yields, and unreacted reagents recovered were as follows:

Gaseous starting materials:

Propylene and pentaborane reagent (at least 80% B H balance B H were reacted in the presence of hydrogen according to the procedure described in Example I. Starting materials, reaction conditions, products and unreacted reagents recovered were as follows:

Gaseous starting materials:

Pentaborane reagent cc 154 Propylene cc 140 Hydrogen cc 28.6 Reaction time hours 3 Temperature C Initial pressure mm 575 Final pressure mm 163 Yield of propylene-B H adduct g 0.5021 Percent yield (based on B H consumed) percent 93 Unreacted materials recovered:

Hydrogen cc 29.1 Pentaborane-ll cc 41.3 Propylene cc 11.7

Example IV Propylene and pentaborane reagent (at least 80% B H balance B H were reacted in the presence of hydrogen according to the procedure described in Example I. Starting materials, reaction conditions, products and unreacted reagents recovered were as follows:

Gaseous starting materials:

Pentaborane reagent cc 164.5 Propylene cc Hydrogen cc 23.8 Volume of reaction tube cc Reaction time hours 5 Temperature C-.. 0 Initial pressure mm 618 Final pressure mm 125 Yield of propylene-13 F1 adduct g 0.527 Percent yield (based on B H consumed) percent 86 Unreacted materials recovered:

Hydrogen cc 23.9 Pentaborane-ll cc 36.7 Propylene cc 8.1

The compositions of our invention can be employed as fuels when burned with air. Thus, they can be used as fuels in basic and auxiliary combustion systems in gas turbines, particularly aircraft gas turbines of the turbojet or turboprop type. Each of those types is a device in which air is compressed and fuel is then burned in a combustor in admixture with the air. Following this, the products of combustion are expanded through a gas turbine. The products of our invention are particularly suited for use as a fuel in the combustors of aircraft gas turbines of the types described in view of their improved energy content, combustion efliciency, combustion stability, flame propagation, operational limits and heat release rates over fuels normally used for these applications.

The combustor pressure in a conventional aircraft gas turbine varies from a maximum at static sea level conditions to a minimum at the absolute ceiling of the aircraft, which may be 65,000 feet or 70,000 feet or higher. The compression ratios of the current and near-future aircraft gas turbines are generally within the range from 5:1 to 15 or 20:1, the compression ratio being the absolute pressure of the air after having been compressed (by the compressor in the case of the turbojet or turboprop engine) divided by the absolute pressure of the air before compression. Therefore, the operating combustion pressure in the co-mbustor can vary from approximately 90 to 300 pounds per square inch absolute at static sea level conditions to about 5 to 15 pounds per square inch absolute at the extremely hi h altitudes of approximately 70,000 feet. The products of our invention are well adapted for efficient and stable burning in combustors operating under these widely varying conditions.

In normal aircraft gas turbine practice it is customary to burn the fuel, under normal operating conditions, at overall fuel-air ratios by Weight of approximately 0.012 to 0.020 across a combustion system when the fuel employed is a simple hydrocarbon, rather than a 'borohydrocarbon of the present invention. Excess air is introduced into the combustor for dilution purposes so that the resultant gas temperature at the turbine wheel in the case of the turbojet or turboprop engine is maintained at the tolerable limit. In the zone of the combustor where the fuel is injected the local fuel-air ratio is approximately stoichiometric. This stoi-chiometric fuel to air ratio exists only momentarily, since additional air is introduced along the combustor and results in the overall ratio of approximately 0.012 to 0.020 for hydrocarbons before entrance into the turbine section. For the higher energy fuels of the present invention, the local fuel to air ratio in the zone of fuel injection should also be approximately stoichiometric, assuming that the boron, carbon and hydrogen present in the products burn to boric oxide, carbon dioxide and Water vapor. In the case of the monoethylpentaborane-ll, for example, this local fuel to air ratio by weight is approximately 0.074. For the higher energy fuels of the present invention, because of their higher heating values in comparison with the simple hydrocarbons, the overall fuel-air ratio by weight across the combustor will be approximately 0.008 to 0.016 if the resultant gas temperature is to remain within the presently established tolerable temperature limits. Thus, when used as the fuel supplied to the cornbustor of an aircraft gas turbine engine, the products of the present invention are employed in essentially the same manner as the simple hydrocarbon fuel presently being used. The fuel is injected into the combustor in such manner that there is established a local Zone where the relative amounts of fuel and air are approximately stoichiometric so that combustion of the fuel can be reliably initiated by means of an electrical spark or some similar means. After this has been done, additional air is introduced into the conbustor in order to cool sufficiently the products of combustion before they enter the turbine so that they do not damage the turbine. Present-day turbine blade materials limit the turbine inlet temperature to approximately l6001650 F. Operation at these peak temperatures is limited to periods of approximately five minutes at takeoff and climb and approximately minutes at combat conditions in the case of military aircraft. By not permitting Operating at higher temperatures and by limiting the time of operation at peak temperatures, satisfactory engine life is assured. Under normal cruising conditions for the aircraft, the combustion products are sufficiently diluted with air so that a temperature of approximately 1400 F. is maintained at the turbine inlet.

The pro-ducts of our invention can also be employed as aircraft gas turbine fuels in admixture with the hydrocarbons presently being used, suchas JP-4. When such mixtures are used, the fuel-air ratio in the zone of the combustor where combustion is initiated and the overall fuel-air ratio across the combustor will be proportional to the relative amounts of borohydrocarbon of the present invention and hydrocarbon fuel present in the mixture, and consistent with the air dilution required to maintain the gas temperatures of these mixtures within accepted turbine operating temperatures.

Because of their high chemical reactivity and heating values, the products of our invention can be employed as fuels in ramjet engines and in afterburning and other auxiliary burning schemes for the turbojet and bypass or ducted type engines. The operating conditions of afterburning or auxiliary burning schemes are usually more critical at high altitudes than those of the main gas turbine combustion system because of the reduced pressure of the combustion gases. In all cases the pressure is only slightly in excess of ambient pressure and efficient and stable combustion under such conditions is normally difiicult with simple hydrocarbons. Extinction of the combustion process in the afterburner may also occur under these conditions of extreme conditions of altitude operations with conventional aircraft fuels.

The burning characteristics of the products of our invention are such that good combustion performance can be attained even at the marginal operating conditions encountered at high altitudes, insuring efiicient and stable combustion and improvement of the zone of operation before lean and rich extinction of the combustion process is encountered. Significant improvements in the nonafter-burning performance of a gas turbine-afterburner combination is also possible because the high chemical reactivity of the products of our invention eliminates the need of flameholding devices Within the combustion zone of the .afterburner. When employed in an afterburner, the fuels of our invention are simply substituted for the hydrocarbon fuels which have been heretofore used and no changes in the manner of operating the afterburner need be made.

The rarnjet is also subject to marginal operating conditions which are similar to those encountered by the afterburner. These usually occur at reduced flight speeds and extremely high altitudes. The products of our invention will improve the combustion process of the ramjet in much the same manner as that described for the afterburner because of their improved chemical reactivity over that of simple hydrocarbon fuels. When employed in a ramjet the fuels of our invention will be simply substituted for hydrocarbon fuels and used in the established manner.

While the present invention has been described with respect to specific embodiments thereof, it is understood that the scope of the invention is limited only by the scope of the appended claims.

We claim:

1. A monoalkyl pentaborane-ll wherein the alkyl group contains from 2 to 4 carbon atoms.

2. Monoethylpentaborane-ll.

3. lflonopropylpentaborane-1l.

4. A method for the preparation of an alkyl pentaborane-ll which comprises reacting pentaborane-ll and a material selected from the group consisting of monoolefin hydrocarbons having from 2 to 4 carbon atoms and mixtures thereof at a temperature Within the range from 25 C. to 30 C. while the reactants are in admixture with hydrogen.

5. The method of claim 4 wherein said mono-olefin hydrocarbon is ethylene.

6. The method of claim 4 wherein said mono-olefin hydrocarbon is propylene.

'7. A method for the preparation of an alkyl pentaborane-ll which comprises reacting pentaborane-ll and a material selected from the group consisting of monoolefin hydrocarbons having from 2 to 4 carbon atoms and mixtures thereof at a temperature within the range from -25 C. to 30 C. while the reactants are in admixture with an inert diluent gas.

No references cited.

TOBIAS E. LEVOW, Primary Examiner.

ROGER L. CAMPBELL, Examiner.

W. F. W. BELLAMY, L. A. SEBASTIAN, R. A. KULA- SON, Assistant Examiners. 

1. A MONOALKYL PENTABORANE-11 WHEREIN THE ALKYL GROUP CONTAINS FROM 2 TO 4 CARBON ATIMS. 